1. Field of the Invention
This invention relates to profile machining apparatus and is particularly although not exclusively applicable to apparatus for machining cam profiles.
2. Description of the Prior Art
Examples of apparatus for machining cam profiles include grinding machines in which a work table carries a motor driven master cam and a component cam axially aligned with and coupled to the master cam to rotate therewith. The work table is arranged to pivot about an axis parallel to the axis of rotation of the master cam. A grinding wheel is driven about an axis parallel to the axis of rotation of the master cam and component to act on a surface of the component cam and a cam follower is mounted with the surface of the follower in fixed relation to the surface of the grinding wheel to engage the master cam. Normally the distance between the pivot axis of the work table and the periphery of the cam follower is arranged so that when the master cam contacts the cam follower at the minimum lift part of the master cam form, that is the base circle, then the axis of rotation of the master cam lies in the plane containing the grinding wheel axis of rotation and contact point between the cam follower and master cam. The rocking table is biased to engage the master cam with the cam follower and the ground profile on the component cam is generated by the rocking motion of the work table as the master cam is caused to rotate whilst in contact with the cam follower. The profile of the ground cam is related to that of the master cam by the following relationships of the generating mechanism;
1. The distance between the axis of rotation of the master cam and the table pivot axis;
2. The distance between the centre distances of the cam follower and wheel relative to the pivot axis;
3. The ratio between the radii of the cam follower and the grinding wheel.
The cam grinding apparatus outlined above has the following disadvantages:
i. The ground work cam profile changes as the grinding wheel radius changes;
ii. One or more master cam forms are required for each work cam to be ground on the component;
iii. The cam follower must be re-positioned on to the correct cam form on the master cam bank as the work table is indexed laterally to the next cam on the cam component.
iv. Each cam form is generated from a different master. That is although the component may have sets of identical forms (e.g. inlet and exhaust cams) each individual cam will be ground from a different master.
v. The cam lift data is stored in the form of machine master cams mounted integrally on the master bank. They are therefore:
1 difficult to modify PA1 2 prone to damage and wear PA1 3 expensive to produce PA1 4 expensive to service/repair.
vi. The master cam forms must be produced on a machine having identical geometry to the machine on which they are to be used.
vii. Although the master cam banks can be inter-changed in order that different component cam shafts can be ground on the same machine, such re-tooling is time consuming and to a great extent limits the flexibility of the machine.
viii. It is virtually impossible to modify the phase relationships between each individual cam on the master cam bank. Such adjustments inevitably require a new master cam bank.
ix. The drive motor is required to rotate both the master cam bank and component cam shaft and also to provide the force for locking the work table against the action of the biasing means as the master cam moves over the cam follower. The motor is required to rotate at varying speeds in order to provide a substantially constant rubbing speed between the grinding wheel and workpiece and at the same time is required to cope with large torque variations in order to cause the rocking of the work table. This latter requirement is especially significant since high contact pressures are required between the master cam and cam follower in order to meet the normal grinding forces.
x. The machine alignments have to be very closely maintained during manufacture and assembly so that master cams can be replaced or inter-changed. Small variations in the generating mechanism geometry between the machine on which the master cam was produced and the machine on which it is to be used will cause significant errors in the forms of the ground component cam.
Recent developments in computer technology have made it possible to generate cam forms from master cam data stored in a computer memory. This eliminates the requirement for a machined master cam bank and the difficulties which arise from the use of the master cam bank as outlined above. Current applications of computers to grinding machines have been for conventional grinding machines adapted to be controlled by the computer so that the wheel head motion is synchronised with the work rotation allowing non-circular profiles to be ground. The angular displacement and velocity of the work and the linear displacement of velocity of the wheel head are maintained by servomechanisms under the control of a micro-processor. However, due to the high velocity and acceleration rate required during cam generation and the large masses and inertias involved (that is the wheel head assembly and feed screw), rotational speed of the work is limited by the response of the control systems (the linear acceleration of the wheel head being proportional to the square of the work rotation speed). Such system have the complication that the grinding feeds and rates must be superimposed on the cam generation motion.